Photothermographic materials which form photographic images by a heat development method are disclosed, for example, by U.S. Pat. Nos. 3,152,904 and 3,457,075, and D. Morgan and B. Shely, Thermally Processed Silver Systems; Imaging Processes and Materials, Neblette, the 8th edition, Sturge, edited by V. Walworth, and A. Shepp, page 2 (1966).
These photothermographic materials comprise a reducible silver source (for example, organic silver salts), a catalytic amount of a photocatalyst (for example, silver halides), and a reducing agent, these components being customarily in a state dispersed into a matrix of an (organic) binder. Although the photothermographic materials are stable at ordinary temperature, silver is formed, after exposure, through an oxidation-reduction reaction between the reducible silver source (acting as an oxidating agent) and the reducing agent with the aid of heat (for example, 80.degree. C. or higher). This oxidation-reduction reaction is promoted by the catalysis of latent images produced by exposure. The silver formed by the reaction of the organic silver salts in exposed areas provides black images to attain image formation in contrast with unexposed areas. Toning agents are used as needed for the photosensitive materials in order to control the image tone of the silver images.
A method so far used most effectively as a conventional technique for antifogging has been to use mercury compounds as antifoggants. Such use of the mercury compounds for the photographic materials is disclosed, for example, by U.S. Pat. No. 3,589,903. However, the mercury compounds encounter an environmental problem, and non-mercury antifoggants have been expected to be exploited. So far, various types of polyhalogen compounds have been disclosed for the non-mercury antifoggants, for example, by U.S. Pat. Nos. 3,874,946, 4,756,999 and 5,340,712, European Patents 605,981A1, 622,666A1 and 631,176A1, JP-B-54-165 (The term "JP-B" as used herein means an "examined Japanese patent publication"), and JP-A-7-2781 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, these compounds described therein have the disadvantages of having only a small effect on the antifogging and deteriorating the image tone of the silver, and further, there also are some compounds which bring about a great effect on the antifogging but cause sensitivity decrease. Thus, improvements in antifogging have been required in this field. Furthermore, when a pile of these photographic materials undergoes aging under forced conditions of high temperatures and high humidities, the photographic materials suffer fog increase in unexposed areas after exposure and development. Thus, antifoggants producing no such troubles have been expected to be exploited